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UC Berkeley UC Berkeley Previously Published Works UC Berkeley UC Berkeley Previously Published Works Title Identifying the Unique Properties of α-Bi2Mo3O12 for the Activation of Propene Permalink https://escholarship.org/uc/item/3z33q49j Journal Journal of Physical Chemistry C, 120(51) ISSN 1932-7447 Authors Licht, RB Getsoian, A Bell, AT Publication Date 2016-12-29 DOI 10.1021/acs.jpcc.6b09949 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Identifying the Unique Properties of α-Bi2Mo3O12 for the Activation of Propene Rachel B. Licht, Andrew “Bean” Getsoian, and Alexis T. Bell* Department of Chemical and Biomolecular Engineering University of California Berkeley, CA 94720-1462 and Chemical Sciences Division Lawrence Berkeley National Laboratory Berkeley, CA 94720 1 Abstract In order to understand the remarkable activity of -Bi2Mo3O12 for selective oxidation and ammoxidation of propene, the propene activation abilityα of four molybdenum-based mixed metal oxides – Bi2Mo3O12, PbMoO4, Bi2Pb5Mo8O32, and MoO3 – was investigated using Density Functional Theory. Propene activation is considered to occur via abstraction of a hydrogen atom from the methyl group of physisorbed propene by lattice oxygen. For each material, the apparent activation energy was estimated by summing the heat of adsorption of propene, the C-H bond dissociation energy, and the hydrogen attachment energy (HAE) for hydrogen addition to lattice oxygen; this sum provides a lower bound for the apparent activation energy. It was found that two structural features of oxide surfaces are essential to achieve low activation barriers: under-coordinated surface cation sites enable strong propene adsorption, and suitable 5- or 6-coordinate geometry at molybdenum result in favorable HAEs. The impact of molybdenum coordination on HAE was elucidated by carrying out a molecular orbital analysis using a cluster model of the molybdate unit. This effort revealed that in 5- and 6-coordinate molybdates, oxygen donor atoms trans to molybdenyl oxo atoms destabilize the molybdate prior to H addition but stabilize the molybdate after H addition, thereby providing an HAE ~ 15 kcal/mol more favorable than on 4-coordinate molybdate oxo atoms. Bi3+ cations in Bi2Mo3O12 thus promote catalytic activity by providing both strong adsorption sites for propene, and by forcing molybdate into 5-coordinate geometries that lead to particularly favorable values of the HAE. 2 1. Introduction Propene oxidation to acrolein and ammoxidation to acrylonitrile are commercially significant processes, each performed on the scale of ~10 billion lbs/year.1 The catalysts used for these processes are based on bismuth molybdenum oxides, though commercial formulations may include as many as 10 additional elements to improve catalyst activity, selectivity, and lifetime.2 The best single-phase catalyst for propene oxidation and ammoxidation is considered to be alpha bismuth molybdate, α-Bi2Mo3O12, though some researchers have suggested that other bismuth 3,4,5 molybdate phases, such as β-Bi2Mo2O9 and γ-Bi2MoO6, exhibit similar activities. Mechanistic studies of propene oxidation and ammoxidation have therefore focused on the three primary bismuth molybdate phases, with particular emphasis on the α-Bi2Mo3O12 phase. The activation energy and partial pressure dependencies for both reactions are the same, leading to the conclusion that the rate-limiting step is the same. 6, 7, 8, 9 Both experimental and theoretical studies have identified this step to be the abstraction of a hydrogen atom from the terminal methyl group to produce a physisorbed allyl radical. 10,11,12,13,14 Isotopic labeling studies have shown that the oxygen atoms involved in propene oxidation come from the catalyst surface and not from adsorbed 15,16, O2, which only serves to reoxidize the catalyst. The elementary steps by which allyl radicals are converted to acrolein or acrylonitrile have been the subject of a number of studies.7,17,18,19,20,21 Considerable effort has been devoted to identifying the characteristics of a mixed metal oxide that make it active for propene oxidation and ammoxidation. While much is known about the structure and electronic properties of bismuth molybdates, the reason why Bi2Mo3O12 is 14,22,23,24,25 particularly active remains a subject of discussion. The crystal structure of Bi2Mo3O12 is related to that of the tetragonal mineral scheelite (CaWO4), but with a monoclinic distortion due 2+ 3+ to the vacancies that result from the replacement of Ca with Bi (Bi2/3φ1/3MoO4 where φ is a 3 vacancy).26 The flexibility of the scheelite structure enables the synthesis of many single-phase, solid-solution tri- and tetra-metal oxides, and has therefore formed the basis for much of this work.27 However, the scheelite structure alone is insufficient for high activity. For example, lead 28 molybdate PbMoO4 crystallizes as a pure scheelite, but is essentially inactive for propene 29 2+ oxidation and ammoxidation. Complete substitution of the Pb , e.g. in Na0.5Bi0.5MoO4, produces a similarly inactive material. However, the activity of these materials increases substantially upon addition of a small amount of Bi3+, with the concurrent addition of vacancies, 27,30 to form Pb1-3xBi2xφxMoO4 (0 < x ≤ 0.07) or Na0.5-3xBi0.5+xφ2xMoO4 (0 < x ≤ 0.17). These findings have led to the conclusion that cation vacancies are required for catalysis and that somehow vacancy sites are responsible for propene activation. It is notable, however, that bismuth addition and vacancy formation occur simultaneously. To decouple the effects of these two parameters, additional experiments have been performed with Pb1-3xLa2xφxMoO4, in which defects are introduced, but not Bi3+ cations.21 The latter system shows no catalytic activity for 0 ≤ x ≤ 0.04, suggesting that both Bi3+ cations and vacancies together are required for a molybdate catalyst to exhibit high catalytic activity. This hypothesis is further supported by observations that europium molybdate31 and cerium or lanthanum molybdate,32 which also crystallize in scheelite- derived structures containing trivalent cations, but with different orderings of cation vacancies, exhibit lower catalytic activities than Bi2Mo3O12. Scheelite structures in which molybdenum is partially or completely replaced with another transition metal (i.e., Fe, V, Ga, W) have also been investigated.33,34 The activation energy of these catalysts for propene oxidation to acrolein correlates very well with the electronic band gap measured by UV-Visible Spectroscopy at reaction temperature or calculated with Density Functional Theory (DFT).22 This correlation was found to be a consequence of the electronic 4 structure of the transition state for H abstraction from propene, which was found to involve a singlet-triplet spin crossing where molybdenyl oxo is rehybridized (Mo=O → Mo–O) so that it can accept the hydrogen atom removed from the methyl group of propene.19,38 The band gap is a measure of the singlet-triplet excitation energy, and therefore correlates to the activation energy for a metal oxo to abstract a hydrogen atom from propene. In these catalysts, the chemistry is presumed to occur on oxygen atoms associated with the redox active metal – molybdenum or its replacement transition metal – and not on oxygen atoms associated with the redox inactive bismuth. This hypothesis is supported by both experimental and theoretical investigation, vide infra. Previous work has shown that while both Bi2O3 and MoO3 are essentially inactive for propene oxidation to acrolein or ammoxidation to acrylonitrile, Bi2O3 is able to activate propene and produce a small amount of hexadiene,35 and molybdenum oxide is able to convert allyl radicals generated in situ to acrolein.36 These findings have led to the longstanding hypothesis that oxygen atoms bound to bismuth are responsible for propene activation to generate allyl radical, while oxygen atoms bound to molybdenum, presumably as molybdenyl oxo groups, are responsible for the conversion of allyl radical to acrolein.37 However, recent XANES studies by our group have shown that this interpretation is not correct. Exposure of Bi2Mo3O12 to pure propene at 713 K for 24 h results in the reduction of molybdenum from the 6+ to the 4+ oxidation state, but bismuth remains in the 3+ oxidation state.18 Supporting DFT calculations19,38 have led us to conclude that oxygen atoms bound to molybdenum atoms perform all of the chemistry in Bi2Mo3O12. Therefore, the question of why bismuth is necessary for catalysis remains unanswered. Our recent theoretical work has shown that Bi3+ cations electronically perturb the neighboring surface molybdenyl oxo groups and, in doing so, enhance the activity of these sites 5 for propene activation via abstraction of a hydrogen atom from the methyl group of adsorbed propene.19 The influence of the Bi3+ cations was attributed to electronic repulsion between the bismuth lone pair and an oxygen 2p lone pair, which destabilizes the Mo=O singlet state, and from back donation from the Mo–O π* orbital to the bismuth 6p orbital, which stabilizes the triplet state. The combination of these electronic effects leads to a lower singlet → triplet excitation energy, and thus lowers the transition-state barrier for hydrogen abstraction from propene. However, the magnitude of this effect is estimated to be only ~3 kcal/mol, which is not enough to explain the overall difference in activity between Bi2Mo3O12 and other molybdenum-oxide-based materials. The preceding discussion demonstrates that both the composition and structure of mixed metal oxides play an important role in defining the activity of such materials for propene oxidation and ammoxidation. The aim of the present investigation is use DFT calculations to identify the extent to which specific geometric and electronic properties contribute to the activation energy for propene activation. We have focused our investigation on oxides containing molybdenum as the only redox-active metal, thereby allowing us to directly compare the effects of crystal structure, Mo coordination, and the identity of additional elements. The four metal oxides investigated are MoO3, Bi2Mo3O12, PbMoO4, and a mixed Pb/Bi molybdate derived from PbMoO4.
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